1. Davy Faraday Research Laboratory, Royal Institution of Great Britain, 21 Albemarle Street, London W1X 4BS, U.K.
2. Department of Chemistry, University of Manchester Institute of Science and Technology, P.O. Box 88, Manchester, M60 1QD, U.K.
3. School of Chemistry, University of St. Andrews, St. Andrews, Fife KY16 9ST, U.K.
4. Department of Chemistry, University of Aveiro, 3800 Aveiro, Portugal